1. Field of the Invention
The present invention relates to an optical device which employs a GRIN rod lens, a single-mode bidirectional optical wavelength demultiplexer/multiplexer which utilizes the optical device, and a bidirectional communications system which utilizes the optical wavelength demultiplexer/multiplexer.
2. Description of the Related Art
Various optical devices can be formed by combining GRIN rod lenses and optical fiber ferrules. A problem with these devices will be described in conjunction with a reference example that the present inventors have studied. FIGS. 4A to 4C are sectional views illustrative of a reference example of an optical device which uses GRIN rod lenses; this device is used for a bidirectional optical divider coupler by employing a pass filter. A pass filter 1 is provided between GRIN rod lenses 2a and 2b which are disposed with the optical axes thereof aligned. The pass filter 1 is characterized in that it reflects a light wavelength .lambda..sub.1 and allows a light wavelength .lambda..sub.2 to pass through; at the central part of a ferrule 4 on the opposite side, two single-mode optical fibers 5 and 6 are inserted and fixed symmetrically with respect to the central axis thereof. An end surface 4a of the ferrule is connected to an end surface of the GRIN rod lens 2a by gluing or by other similar means.
A ferrule 8 on a transmitting end supports an optical fiber 9; the ferrule 8 on the transmitting end is connected to a GRIN rod lens 21b through an end surface 8a thereof so that the optical fiber 9 may receive a transmitted light .lambda..sub.2 of the pass filter 1.
This optical wavelength demultiplexer/multiplexer is characterized by its large isolation; an isolation value of 40 dB or more can be easily achieved in this reference example while the isolation value of an optical wavelength demultiplexer/multiplexer manufactured by a heat drawing process is approximately 15 dB. Achieving a minimized connection loss, however, requires precise, utmost control during the manufacturing process. Specifically, as shown in the enlarged sectional view, precise symmetrical positioning must be attained so that the positional relationship between the optical axes of the GRIN rod lenses and the optical axes of all the single-mode optical fibers substantially satisfies a requirement of S.sub.1 =S.sub.2 =S.sub.3. Further adjustment is required to assure that the optical axes are accurately positioned on a line X-X' on XY rectangular coordinates.
When using the single-mode optical fibers, connection loss IL in dB caused by misaligned optical axes is defined as follows: ##EQU1## where d: Dislocation of optical axes of optical fibers from those of GRIN rod lenses EQU d=S1-S2 or d=S1-S3
.omega.: Mode field radius of fiber core
About 4 .mu.m for a typical single-mode optical fiber PA2 About 25 .mu.m for a typical multi-mode optical fiber
In the case of the aforesaid typical single-mode optical fiber, the insertion loss is 1.09 dB when .omega.=4 .mu.m and dislocation of optical axes is d=2 .mu.m.
A different formula is used for the multi-mode optical fiber; however, if the same formula is approximatively applied, then a permissible dislocation (d) of optical axes resulting in the same insertion loss would be 12.5 .mu.m. This means easier adjustment.
The single-mode optical fibers are extensively used mainly because they permit broader-band transmission and lower transmission loss. As described above, however, it is not easy to accurately align the cores in assembling an optical wavelength demultiplexer/multiplexer for single-mode optical fibers and advanced techniques and many man-hours are required. For this reason, the optical wavelength demultiplexer/multiplexer for single-mode optical fibers has not been widely disseminated despite the good optical performance thereof.